A primary task in producing conventional liquid crystal display (LCD) devices was to make a greatest possible contrast, since a greater monochromic contrast was demanded. However, as LCD devices have been incorporated in multimedia apparatuses recently and the quality of picture of personal computer software has been enhanced due to CD-ROM (Compact Disk Read Only Memory) or the like, to achieve regularity of display in half tone, that is, how to obtain half tone display without irregularity, has become an important task. The quality of such half tone display is greatly affected by alignment of liquid crystal, and one of factors causing irregular alignment of liquid crystal is an irregular alignment operation on an alignment film. The following description will depict the conventional alignment operation.
A liquid crystal display (LCD) device-use substrate is composed of a glass substrate, transparent electrodes provided on the glass substrate, and an alignment film which is laminated over the glass substrate so as to cover the transparent electrodes. In the process of producing such an LCD device-use substrate, a rubbing device is adopted as an alignment device. As shown in FIG. 9, such a rubbing device is provided with a rubbing roller 30 having a rotary 30a and a rubbing cloth 31. The rubbing cloth 31 is provided with innumerable piles P thereon, and is provided on the circumferential surface of the rotary roller 30a. An LCD device-use substrate is provided near the rubbing roller 30, so that the surface of the alignment film is rubbed in a specific direction by rotating the rubbing roller 30. With such an arrangement, the liquid crystal molecules are aligned in the rubbing direction. Generally, such an operation for rubbing an alignment film in a specific direction is called an alignment operation.
The rubbing cloth 31 is formed by interweaving pile material composed of filament bunches into foundation warp and weft yarns, each bunch being composed of several tens of filaments. The piles P are cut at a uniform length. Thus, the rubbing cloth 31 is arranged so that filaments densely touch the LCD device-use substrate. As shown in the figure, usually a rectangular rubbing cloth is attached to the circumferential surface of the rotary roller 30a, with the edges of the rubbing cloth 30a parallel with the edges of the rotary roller 30a's circumferential surface.
Other methods of providing a rubbing cloth around the rotary roller are disclosed in the Japanese Publication for Laid-Open Utility Model Application No. 2-33030/1990 and the Japanese Publication for Laid-Open Patent Application No. 54-40652/1979.
According to the Japanese Publication for Laid-Open Utility Model Application No. 2-33030/1990, as shown in FIG. 10, a rubbing cloth 33 in a ribbon form is spirally wound around the circumferential surface of the rotary roller 32a, so that the edges of the rubbing cloth 33 where piles P are not provided is tilted to the rotation axis of the rotary roller 32a. This was invented in the light of the following problem arising from the arrangement of the rubbing roller 30 shown in FIG. 9. Since the junction part of the rubbing clothes 31, with a width of 1 through 2 millimeters, does not have piles P, an alignment operation with use of the rubbing roller fails to align a part of the alignment film which corresponds to the junction part having no piles P. Therefore, a linear unaligned region is formed with every rotation of the rubbing roller 30.
According to the Japanese Publication for Laid-Open Patent Application No. 54-40652/1979, rubbing cloth material such as a cotton cloth or an abrasive cloth is cut out into a diamond shape. The rubbing cloth thus cut out is fixed around a circumferential surface of a rotary roller with cushion material such as sponge therebetween, so that the junction part of the rubbing cloth is tilted to the rotation axis direction of the rotary roller. This was invented so as to solve the following problem. If the rubbing cloth is rectangular, the joint part of the rubbing cloth is provided in parallel with the rotation axis direction of the rotary roller. Therefore, when the rubbing roller rotates and rubs an alignment film, it likely causes a linear unaligned portion. The above-described arrangement attempts to solve this problem by dispersing, in the oblique direction, such a defect caused by the junction part.
With the arrangement of the rubbing roller 30 shown in FIG. 9, however, the piles P unevenly touch the alignment film due to the positions of the piles P on the rubbing cloth 31. As a result, an LCD device, incorporating the LCD device-use substrate to which an alignment operation has been carried out with the rubbing device having the rubbing roller 30, tends to have irregularity in the display when the LCD device is in the ON state. To be more specific, the display has long and thin stripes running in the rubbing direction (hereinafter such irregularity caused in the display due to the uneven alignment is referred to as "irregular display with stripes"). This leads to irregularity in half tone display. Thus, this arrangement presents a problem of low quality concerning display.
The following description will discuss the cause of the problem, referring to FIG. 4 which explains the present invention. A rubbing cloth is formed by interweaving pile material composed of bunches of filaments F into foundation warp and weft yarns and cutting the piles at a uniform length, each bunch being composed of several tens of filament. The several tens of filaments F, composing one pile, flare out from the root to the end. Therefore, when the filaments F have the same length, the touch of the peripheral part of one pile P is weaker than that of the central part of the pile P. The difference between the strengths of their touches corresponds to the difference between the apparent length L.sub.2 of the peripheral part of the pile P and the length L.sub.1 of the central part of the pile P, namely, L.sub.1 -L.sub.2.
Therefore, with the rubbing roller 30 arranged so that the rubbing cloth 31 is cut out into a rectangular shape in the same size as that of the circumferential surface of the rotary roller 30a and is fixed therearound with the edges of the rubbing cloth 31 in parallel with the edges of the circumferential surface of the rotary roller 30a, the piles P are parallel and orthogonally lined with respect to the rubbing roller 30. With this arrangement, during an alignment operation, the central parts of the piles P touch the alignment film in lines at certain intervals. As a result, the surface of the alignment film is aligned unevenly, due to the difference in the strengths of the touches of the piles P's central and peripheral parts. When an LCD device is produced through such an alignment operation, the "irregular display with stripes" due to the uneven alignment is observed when the LCD device is in the ON state, as shown in FIG. 11(a), 11(b), or 11(c).
The uneven alignment seriously affects the super twisted nematic (STN) LCD device in particular, since the display quality of the STN LCD device greatly depends on the alignment condition. When characteristics are enhanced so as to achieve high-quality display with the improvement of the quality of picture, the irregularity in the half tone display, that is, stripes irregularly appearing on the display, tends to stand out, thereby causing the yield to fall.
Note that although the Japanese Publication for Laid-Open Utility Model Application 2-33030/1990 and the Japanese Publication for Laid-Open Patent Application 54-40652/1979 disclose a method of providing the rubbing cloth around the rotary roller with a tilt, they mention nothing except simply providing them with a tilt. They mention nothing about the relation between the direction of the piles' lines on the rubbing cloth and the rotation direction of the rubbing roller. Therefore, the arrangements disclosed in the above-mentioned publications by no means solve the above-described problem.
On the other hand, the Japanese Publication for Laid-Open Patent Application 2-22624/1990 discloses still another arrangement. According to the arrangement, during a rubbing operation, the rotation axis direction is tilted at an angle falling in the range of .+-.1.degree. through .+-.45.degree. to the orthogonal direction of the moving direction of the LCD device-use substrate. When the end of each pile on the rubbing cloth is split, it tends to cause irregular alignment, which results in comparatively strongly aligned linear portions and comparatively weakly aligned linear portions, and the described arrangement intends to prevent such a phenomenon. However, in this case, since the rotation direction of the rubbing roller 34 and the moving direction of the LCD device-use substrate 35 differ, the surface of the alignment film is curvedly rubbed as shown in FIG. 12(a), thereby causing an inconvenience that the twist angles of the liquid crystal slightly vary. Note that when the rubbing roller 34 rotates in the moving direction of the LCD device-use substrate 35 as shown in the FIG. 12(b), the rubbing is carried out straight in the moving direction, and the twist angles of the liquid crystal are not adversely affected.